1. The Field of the Invention
The present invention relates to tunable vertical cavity surface emitting lasers. More particularly, the present invention relates to tunable vertical cavity surface emitting laser using an electrostrictive polymer that includes a holographic optical element with a narrow filter function.
2. Background and Relevant Art
Vertical cavity surface emitting lasers (VCSELs) are an example of semiconductor lasers that are used in optical fiber systems. VCSELs have several advantages over other types of semiconductor lasers. VCSELs can be manufactured in large quantities due to their relatively small size and can often be tested on a single wafer. VCSELs typically have low threshold currents and can be modulated at high speeds. VCSELs also couple well with optical fibers.
VCSELs typically emit wavelengths on the order of 0.85 microns. VCSELs that operate at single wavelengths or at longer wavelengths on the order of 1.3 to 1.55 microns, which are more useful in optical communications systems, are very difficult to manufacture or fabricate. The difficulty in fabricating VCSELs that generate light in a single mode and/or at longer wavelengths is often related, for example, to the atomic lattice structure of the materials, the quality of the active region or gain medium, the reflectivity of the mirror systems, and the material composition.
Another problem with VCSELs is related to their tunability. Tunable semiconductor lasers are very useful, especially in wavelength-division multiplexing (WDM) systems. When fixed wavelength lasers are used in WDM systems, it is necessary to have a separate VCSEL for each wavelength. For example, a 100 channel WDM system requires 100 different VCSELs. This leads to a number of different problems from maintaining an adequate inventory for spare parts to producing and testing VCSELs of varying wavelengths. A tunable laser can alleviate many of these expensive issues.
In general, tunable lasers often suffer from needing a long gain cavity in order to generate sufficient gain. From a tuning perspective, the long cavity is extraneous and complicates the tuning functionality because the modes of the gain cavity, for example, must be kept in a fixed relationship with respect to the tuning clement. Thus, many tunable lasers require a phase adjust section. VCSELs have the desired short cavities, but the gain for single mode VCSELs is insufficient to allow generation of optical power in the multi-milliWatt range. Increasing the diameter of the VCSEL aperture to increase power also results in multi mode emission.
These and other limitations are addressed by the present invention, which relates to a tunable vertical cavity surface emitting laser that emits a single mode. The single mode is emitted by forming a photonic crystal in the VCSEL that is highly reflective for certain wavelengths. As a result, only the reflected mode(s) achieves appreciable gain in the active region of the VCSEL. The photonic crystal also includes a central defect for propagation of the emitted light. The tunablity is achieved by forming an electrostrictivc material on the VCSEL. A reflection hologram or holographic optical element (HOE) with a narrow filter function is included in the electrostrictive material. The central wavelength of the HOE can be tuned by applying an electric field or voltage to the electrostrictive material. Advantageously, the tunable VCSEL can emit a single mode over a tunable range and higher power at longer wavelengths.
In one embodiment of the present invention, a photonic crystal with a central defect is formed on the upper DBR layers of a VCSEL, or a periodic cavity structure is formed directly in the DBR layers. Alternatively, the DBR layers are omitted and the photonic crystal with the central defect is formed directly on the active region. The reflectivity of the photonic crystal is dependent on the wavelength of the light and on the angle of incidence. The photonic crystal provides the necessary reflectivity such that a single mode is reflected through the active region, which results in stimulated emission of photons at the corresponding wavelength of the incident photon. Usually, the central defect corresponds to an aperture through which the laser light is emitted from the VCSEL.
After the photonic crystal is formed, a tuning element is also formed on the photonic crystal. The tuning element is an electrostrictive polymer that includes a reflection hologram or HOE with a narrow filter function. The electrostrictive polymer undergoes dimensional changes when an electric field is applied and the center wavelength of the HOE is thereby tunable. The HOE also has high reflectivity and the DBR layers may be omitted in one embodiment.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.